Technical Abstract:
In barley, the Mla13 powdery mildew resistance gene confers Rar1-dependent, AvrMla13-specific resistance to Blumeria graminis f. sp. hordei (Bgh). We have identified cDNA and genomic copies of Mla13, and used these as a model for the regulation of host resistance to obligate biotrophic fungi. We demonstrate quantitatively that transcription of Mla6 and Mla13, as well as the Mla-signaling pathway components, Rar1 and Sgt1, are induced at 16 hours post inoculation, the same time frame that haustoria of avirulent Bgh make contact with the host cell plasma membrane. Alternative splicing of two introns within the transcript leader region (TLR) of Mla13 can lead to a different number of upstream open reading frames (uORFs) and variability in the size of uORF2. The presence of the Mla TLR, which regulates levels of translated reporter protein, appears to be required for proper activity in vivo since Mla6 with a deleted TLR, under the control of a strong promoter, leads to a loss of resistance specificity. Our results indicate that regulation of Mla transcription is not constitutive and that induction is coordinately controlled by recognition-specific plant factors. The sudden increase in transcription could account for the rapid defense response phenotype conferred by Mla6 and Mla13, while simultaneous control of the amount of protein synthesis would prevent uncontrolled cell death.